Conventional low density polyethylene has been historically polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. The molecular structure of high pressure, low density polyethylene (HP-LDPE) is highly complex. The permutations in the arrangement of their simple building blocks are essentially infinite. HP-LDPE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. HP-LDPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length. These short chain branches disrupt crystal formation and depress resin density.
With recent developments in low pressure technology, low density narrow molecular weight distribution linear ethylene polymers can now be produced at low pressures and temperatures by copolymerizing ethylene with various alphaolefins. These low pressure LDPE (LP-LDPE) resins generally possess little, if any, long chain branching. They are short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
U.S. Patent application Ser. No. 892,325 filed Mar. 31, 1979, now abandoned and refiled as Ser. No. 014,414 on Feb. 27, 1979, in the names of F. J. Karol et al. and entitled Preparation of Ethylene Copolymers In Fluid Bed Reactor, and which corresponds to European Patent Application No. 79100953.3 which was published as Publication No. 004,645 on Oct. 17, 1979, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are copolymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material.
U.S. patent application Ser. No. 892,322 filed Mar. 31, 1978, now abandoned and refiled as Ser. No. 012,720 on Feb. 16, 1979, in the names of G. L. Goeke et al. and entitled Impregnated Polymerization Catalyst, Process For Preparing, and Use For Ethylene Copolymerization, and which corresponds to European Patent Application No. 79100958.2 which was published as Publication No. 004,647 on Oct. 17, 1979, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities, if the monomer(s) are copolymerized in a gas phase process with a specific high-activity Mg-Ti containing complex catalyst which is impregnated in a porous inert carrier material.
U.S. patent application Ser. No. 892,037 filed Mar. 31, 1978, now abandoned and refiled as Ser. No. 014,412 on Feb. 27, 1979, in the names of B. E. Wagner et al and entitled Polymerization Catalyst, Process For Preparing And Use For Ethylene Homopolymerization, and which corresponds to European Patent Application No. 79100957.4 which was published as Publication No. 004,646 on Oct. 17, 1979, discloses that ethylene homopolymers having a density of about .gtoreq.0.958 to .ltoreq.0.972 and a melt flow ratio of about .gtoreq.22 to about .ltoreq.32 which have a relatively low residual catalyst residue can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process if the ethylene is homopolymerized in the presence of a high-activity Mg-Ti-containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced are useful for a variety of end-use applications.
The polymers as produced, for example, by the processes of said applications using the Mg-Ti containing complex catalyst possess a narrow molecular weight distribution, Mw/Mn, of about .gtoreq.2.7 to .ltoreq.4.1.
Over the years, film extrusion equipment has been optimized for the rheology of HP-LDPE. The different molecular architecture of low pressure-low density polyethylene (LP-LDPE) result in a film processing behavior which requires different extrusion parameters. By way of background, conventional extruder screws commonly used for HP-LDPE include an elongated barrel which may be heated or cooled at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a helical land on its surface which cooperates with the cylindrical internal surface of the barrel to define an elongated helical channel. Although the pitch of the screw may vary along the length thereof, it is common at the present time to utilize screws of constant pitch wherein the pitch is "square", that is, where the distance between adjacent flights is equal to the diameter. The screw is rotated about its own axis to work the plastic material and feed it toward the outlet end of the barrel.
An extruder screw ordinarily has a plurality of sections which are of configuration specially suited to the attainment of particular functions. Examples are "feed" sections and "metering" sections, which are of basic importance and are present in nearly all extruders for handling thermoplastic polymers.
A typical extruder screw feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel by the feed section of the srew. In this section the channel depth of the screw is usually large enough to over-feed the solid polymer. This is a desirable effect because the over-feeding action serves to compact and pressurize the polymer particles and form a solid bed of advancing material.
The working of the material generates heat, and melting of the polymer proceeds as the material is moved along the feed section of the screw. Actually, most of the melting occurs near the barrel surface at the interface between a thin melt film and the solid bed of polymer. This general pattern persists until a substantial portion of the polymer reaches the melted state. After some 40 to 70 percent of the polymer has been melted, solid bed breakup usually occurs, and at this time particles of solid polymer become dispersed in the polymer melt. From this point on, it often is advantageous to intimately mix the polymer melt with the unmelted material to accelerate melting and minimize local non-uniformities.
An extruder screw "metering" section has as its special function the exertion of a pumping action on the molten polymer. Ordinarily the throughput achieved by a screw is thought of as being a function of the combination of the "drag flow" and "pressure flow" effects of the metering section.
Drag flow is basically the flow which results from the relative movement between the screw and the internal surface of the extruder barrel. It may be thought of as being proportional to the product of the average relative velocity and the channel cross-sectional area. This drag flow component is directed toward the outlet end of the screw. It may be increased by increasing the speed of the screw and/or by increasing the depth of the flow channel in the screw.
Acting in opposition to drag flow is a pressure flow component stemming from the reluctance of the material to flow through the restricted outlet opening at the end of the extruder passage. The speed of the screw does not directly affect the pressure flow component but of course it may affect such factors as back pressure and material viscosity, which factors, in turn, affect significantly the pressure flow component. On the other hand pressure flow is directly affected by both the depth and length of the screw channel; an increase in channel depth has a tendency to increase greatly the pressure flow component and an increase in channel length has a tendency to reduce this back flow component.
In addition to the basic "feed" and "metering" sections an extruder screw also may include a number of other distinct sections. Nearly all screws include, for example so-called "transition" sections.
Over the years there has been a trend toward the use of extruders capable of high outputs. In many applications, various economies in production are possible where high extruder outputs can be obtained on a reliable basis.
Although LP-LDPE resins can be extruded on equipment designed for HP-LDPE resins, such as described above, certain equipment modifications are often required in order to extrude the low pressure resins at optimum conditions and at rates comparable to the high pressure resins. This is particularly true during extrusion of LP-LDPE which is subsequently processed into film. The problem appears to be that when the new granular, low pressure resins are extruded through equipment designed for the earlier high pressure resin pellets, they tend to run at lower throughput rates. When they are pushed to higher rates, the operation becomes unstable. An analysis of the problem revealed that the above deficiencies resulted from starvation, i.e. incomplete filling of the metering section of the extruder screw due to the different solids and melt rheology of the LP-LDPE.
Accordingly, the present methods for extruding LP-LDPE resins are not entirely satisfactory from a practical commercial standpoint and there currently exists a need for providing an extrusion method which provides additional compression (needed with granular materials because of lower bulk densities) without inordinately deepening the feed section and weakening the screw used for extrusion.